While very basic technologies such as land & grass filtration, septic tanks/soak-away pit and stabilization ponds are still widely and appropriately used in many developing countries, these technologies are only able to meet non-stringent discharge standards. More advanced technologies must be used to meet increasingly more stringent effluent discharge requirements. These advanced technologies are based on the use of microbial activity under aerobic and/or anaerobic and/or anoxic treatment conditions to meet different and multiple treatment objectives. Treatment objectives range from the removal of simple organics (biological oxygen demand [BOD], chemical oxygen demand [COD], total organic carbon [TOC]) and total suspended solids [TSS] removal to the meeting of stringent nutrient (nitrogen [N], phosphorus [P]) discharge standards and the removal of complex organics and/or toxic contaminants.
Biological wastewater treatment technologies may be classified using a number of categories that relate to the presentation of the biomass, flow, operation, configuration. etc. of the treatment process.
These categories include, but are not limited to:                Flow: dispensing of the wastewater into, out of, and within the system, which may be a continuous or intermittent process;        Operation: the control and operation of the system, which may be continuous or intermittent for the operational parameters, e.g. flow, volume, aeration, temperature, pH, mixing, recycling, excess sludge wastage rate, etc.        Biomass: micro-organisms that are used for the removal of waste material. The biomass may be attached as a biofilm on the surface of a carrier or cultured in a suspension generally known as activated sludge;        Sludge: used to refer to the different types of biomass in the reactor, e.g. activated sludge, waste sludge, nitrifying sludge. For this invention, the term “sludge” is used with emphasis on its uniqueness in achieving treatment objective(s) and is in the form of biofilm when it is fixed on a carrier, and/or activated sludge when it is suspended in the liquid.        Carrier: support medium (numerous types) used for attachment of a film of biomass. The carrier may be made of different materials (examples include but are not limited to wood chips, gravel, ceramics, alloy, plastics, rubber, recycled tyres), and may be mobile (i.e. freely movable within the tank) or fixed (i.e. immobilized or attached to the tank with limited movement)        Tank: major physical structure(s) for containment of liquid. A tank may also have the same meaning as reactor or bioreactor. Reactors may be sub-divided into tanks, and the tanks into sub-tanks. Other similar terms such as “basin”, etc. will not be used herein for clarity.        Stage: the predominant biochemical or bioreaction function in the pollutant removal process, for example removal of carbonaceous, nitrogenous (nitrification function and/or denitrification function, etc.) or phosphorous compounds. In any system, there may be different tanks or tanks/sub-tanks that are used to perform different stages of pollutant removal. These are typically configured and operated differently from the other stage(s) to achieve the predominant function.        Reactor: Tanks of the wastewater treatment plant, which may include chemical, physical, biological, etc. processes.        Clarification: process wherein the biomass is separated from the water to produce the final effluent, e.g. using secondary clarifier, SBR during settling phase, etc.        Bio-selector: optional initial stage where the biomass first contacts the wastewater wherein conditions of initial high food: microorganism ratio (F/M) or floc-load are established to enhance the biomass characteristics, particularly minimization of bulking and foaming bacteria. Many types (e.g. aerobic, anoxic, anaerobic or their combination) and designs are available.        
Each combination has various advantages and disadvantages that may make them more suitable, space-efficient and cost-effective for one or another application.
Current Available Technologies: Aerobic (Full or Partial) Treatment Processes Activated Sludge Processes
Activated sludge (AS) processes are the currently most widely used treatment systems. They are capable of meeting very high effluent BOD/COD and nutrient discharge standards. Wastewater is mixed with suspended biomass, and the resulting mixed liquor typically flows continuously through the treatment system. Thus, the AS is subjected to different conditions. The pollutants are converted to solids (biomass or sludge) and/or gas with the production of water through their metabolic processes and under the following typical conditions as regards oxygen species:    1. Organic removal only: under fully aerobic conditions;    2. Organic and nitrogen removal only: under aerobic and anoxic conditions;    3. Organic, nitrogen and phosphorus removal: under aerobic, anoxic and anaerobic conditions.
Following biological wastewater treatment the biomass must be separated from the wastewater so that treated effluent may be discharged. This separation is normally done using gravity sedimentation or forced clarification. To facilitate the separation process, the operating mixed liquor suspended solids (MLSS) concentration is typically restricted to <3,500 mg/L. The excess sludge produced, as the end product of biological wastewater treatment, requires further treatment, usually digestion and/or dewatering.
Growth of slow growing and sensitive bacteria such as nitrifiers and those required to break down complex refractory organics is a rate-limiting step that requires long operating sludge ages to achieve the desired effluent quality. The resultant low food:microorganism ratio (F/M) results in large volume bioreactor tanks. For nutrient removal, additional anaerobic/anoxic tanks must be added.
The secondary clarifiers also have large volume requirements, particularly if the operating sludge age is long, which typically results in poorly settling sludge. Handling of the excess sludge also requires substantial capital investment and higher operating costs. The tanks and clarifiers also require large areas for their construction. In highly populated regions, land limitations restrict the feasibility of using conventional activated sludge with clarifier systems.
Sequencing Batch Reactor (SBR) Activated Sludge Processes
To eliminate separate secondary clarification units, fill-and-draw or batch treatment systems such as the Sequencing Batch Reactor (SBR) were revived. In these SBR systems, which use activated sludge and cyclic time-oriented ON/OFF operation, the entire treatment tank is also used as a clarifier. A high degree of process control of all unit operations enables high treatment standards to be met.
In the second generation of SBR technology (SBR2), floc-load controlled bio-selector and separate anaerobic/anoxic tanks, similar to those of the conventional continuous flow AS systems, were introduced to provide filamentous bulking and foaming control and better nutrient removal, respectively. However, the SBR systems are still hydraulically limited with resultant large reactor tank(s) by the need for long operating sludge ages as restricted by the large unaerated mass fraction of typically 50% which result in large MLSS requirements and, consequently, a long time for settling of biomass and decant of treated effluent. Typical hydraulic retention time (HRT) ranges from 15-24 hours. Furthermore, during part of each cycle the decanters and the aeration diffusers are inactive giving, in effect, a fraction of ‘inactive capital’.
Bio Film Processes
Biofilm systems are attached biomass systems that use a solid support medium or carrier(s) on which the biomass grows. Excess biofilms falls off the carriers such that a secondary clarifier (small) is still needed. Conventional biofilm systems include trickling filters, rotating biological contactors (RBC) and submerged aerated filters (SAF). These can be compact systems, but suffer from poor control of reaction conditions, problems with mixing and oxygen transfer, slow start-up and recovery times from upset, clogging, and a very complex ecology. Newer systems include the biological aerated filters (BAF), which rely on backwash of the fixed media to remove the excess biofilm.
Hybrid Processes—Combined Biofilm and Activated Sludge
In these relatively recent processes, both carriers (fixed or mobile) and activated sludge are in the same treatment system. Fixed carrier AS systems, such as activated trickling filters and aerated RBC systems, are used mainly for high strength wastewater treatment. They consist of fixed carrier biofilm systems with a downstream AS system to meet the required effluent discharge standards. In contrast, mobile biofilm carriers are incorporated in AS systems to treat low strength wastewaters for nitrification with and without denitrification. These technologies include moving bed bioreactor (MBBR) and sequencing biofilm batch reactor (SBBR), which consist of simply adding carriers to the reactor tank.
These hybrid processes offer advantages including a more compact footprint (smaller process volume) due to the independence of the HRT from the operating sludge age. However, these hybrid systems also suffer from some of the same problems as the biofilm technologies, particularly, poor control of reaction conditions.
Current Available Technologies: Anaerobic Treatment Processes
Anaerobic treatment is a process that involves (1) the biological hydrolysis of particulates in the wastewater to soluble organic matter followed by conversion of soluble organics to short chain organic acids and (2) the production of gas (methane and carbon dioxide), all in the absence of oxygen.
Anaerobic treatment is suitable to treat high strength industrial wastewaters and side-stream(s) of large municipal sewage treatment works. However, meeting typical effluent discharge requirements is difficult and there is no nutrient removal capability. A downstream aerobic treatment is sometimes used. The methane produced may be used to generate energy.
Low technology systems (low reaction rate/large volume) have historically been used for a range of applications. Typical current-day applications include digestion of sludges and solid waste, and pond treatment. Two-stage high-rate anaerobic processes have been developed in recent years with higher reaction rates and low HRT (e.g. one day). High-rate treatment is widely used for high strength soluble organics industrial waste, and occasionally for the treatment of sewage.
In the first stage, incoming organic carbon is converted to small chain organic acids in a continuously fed, stirred-tank reactor. In the second stage, the acids are converted to methane and carbon dioxide gas. The organisms generally grow together in flocs or on artificial media, and are relatively slow growing and pH sensitive. Operating temperatures may be either in the thermophilic or mesophilic range.
Many anaerobic reactor designs are available, including Upflow Anaerobic Sludge Blanket (UASB), Contact (or Internal Circulation) Reactor, Fixed Film/Bed Reactor, Hybrid, Fluidised Bed (FB) and Expanded Granular Sludge Blanket (EGSB). The main technical challenge and focus has been stability of the second stage, but little (if any) attention has been made to improvements in the first stage. A common limitation is poor control of reaction conditions.
It is therefore the object of this invention to provide a new and improved method of wastewater treatment using a new hybrid SBR technology.